You're being either overly generous to Intel or you're underestimating how well funded some of the espionage organization really are. And consider that for decades espionage organizations have been taking advantage of bugs in commercial products. This is so well documented at this point that it's not even worth arguing. Personally I believe this has been going on since the '70's.

Well-funded espionage organizations can exploit almost everything, so almost everything is a backdoor? Is that a useful definition?

"Thieves can break windows so there's no need to lock your doors". Fallacy.

What is a backdoor? A set of secret instructions that lets you get control or access over target machine. Exactly what this is. Re-read the README.